Unfortunately the babel minify-mangle plugin seems to be abandoned and
in certain circumstances can be very buggy. The only safe options are to
disable it, or to switch to a different minification library.
Not minifying our javascript is not ideal, so this commit updates the
javascript tasks to use a rollup, combined with babel, and terser.
Babel still converts code from ES/UMD/AMD to AMD modules with the
relevant browser support, whilst terser minifies the code.
The rollup bundler handles tracking and creation of sourcemaps, and
supports better parallelisation of the tasks.
Since the upgrade to Node LTS/Gallium requires an upgrade to @babel/core
and eslint, which change the built files anyway, this seems like the
ideal time to make this change.
A previous change as part of this commitset was intended to move the
modal to the document.body, but where the modal had been destroyed it
was instead added back to the body.
Before this change when a modal was shown and an element on the page
was fullscreen the modal would be created behind it.
This change ensures that the modal will be inside an element that is
fullscreen so that it will be correctly displayed.